Fix unused_assignments false positives from macros

compiler/rustc_mir_transform/src/liveness.rs

@@ -72,6 +72,11 @@ pub(crate) fn check_liveness<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Den
         return DenseBitSet::new_empty(0);
     }
 
+    // Don't run unused pass for items generated by foreign macros
+    if tcx.def_span(parent).in_external_macro(tcx.sess.source_map()) {
+        return DenseBitSet::new_empty(0);
+    }
+
     let mut body = &*tcx.mir_promoted(def_id).0.borrow();
     let mut body_mem;
 

tests/ui/liveness/auxiliary/aux_issue_147648.rs

@@ -0,0 +1,7 @@
+#[macro_export]
+macro_rules! unused_assign {
+    ($x:ident) => {
+        let mut $x = 1;
+        $x = 2;
+    };
+}

tests/ui/liveness/unused-assignments-from-macro-147648.rs

@@ -0,0 +1,10 @@
+//@ check-pass
+//@ aux-build:aux_issue_147648.rs
+
+#![deny(unused_assignments)]
+
+extern crate aux_issue_147648;
+
+fn main() {
+    aux_issue_147648::unused_assign!(y);
+}